In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
Currently mainly two solutions for holographic storage systems are discussed. In the collinear system, as disclosed for example in EP 1 624 451, separate parts of the objective lens aperture are used for the object beam and the reference beam, respectively. This system uses a kind of shift multiplexing as a multiplexing method. In the off-axis recording system, as disclosed for example in U.S. Pat. No. 6,721,076, the object beam and the reference beam do not share the same optical path. In this system angle and polytopic multiplexing are used.
For both solutions the overlapping of the reference beam and the object beam in the holographic storage medium is not optimal. This has the consequence that a significantly lower data density is obtained.
In holographic data storage the achievable capacity is strongly related to the so called M-number (M#) of the storage material. This number can be calculated as M#=M·√{square root over (η)}. Here M denotes the number of holograms that can be multiplexed with a given diffraction efficiency □. This equation can only be applied when the overlap of the object beam and the reference beam is optimal. In practice, the overlap is always only partial. Thus a factor Lol has to be introduced, which describes the material consumption of the non-overlapping beams, so that M#=M√{square root over (η)}·Lol. The factor Lol is about 2 for collinear and off-axis recording. For further details see Curtis et al., “M/# requirements for Holographic Data Storage”, Proceedings of the ODS conference 2006, pp. 9-11. In conclusion, a non-optimum overlap of the object beam and the reference beam in the holographic storage medium decreases the achievable capacity if a specific material and optical setup is given.